Conventionally, carbon monoxide is obtained during the steam reforming or partial oxidation of hydrocarbons. It is possible to produce highly pure carbon monoxide with such units by using conventional purification techniques. These include cryogenic processes such as partial condensation or washing with liquid methane; further processes employed for purification are absorption processes, such as the Tenneco Chemicals COSORB process, or adsorption processes (Pressure Swing Adsorption or PSA).
In particular, carbon monoxide is produced during preparation of the synthesis mixture used for the synthesis of ammonia, when the hydrogen is obtained by partial oxidation of hydrocarbons or coal. Such a conventional process of preparing the synthesis mixture is illustrated in the diagram in FIG. 1 and will be described in brief below:
After removal of the soot at 1, and removal of the hydrogen sulphide at 2, from the mixture delivered by the step of partially oxidizing hydrocarbon or coal at 3, step 4 of converting the CO changes the majority of the CO present, under the action of steam, into carbon dioxide (CO.sub.2) while producing hydrogen.
After removal of CO.sub.2 by absorption at 5, the gas mixture is subjected to a cryogenic step 6 of washing with nitrogen, which provides the synthesis mixture (N.sub.2 +3H.sub.2) as well as a residual gas containing essentially carbon monoxide, hydrogen, methane and nitrogen.
FIG. 1 also represents the other operations in the synthesis of ammonia: air distillation at 7, providing the oxygen needed for the partial oxidation 3 and the nitrogen needed for the washing 6; NH.sub.3 synthesis proper at 8, from the synthesis mixture output by step 6, compressed at 9; steam expansion in a turbine 10 to drive the compressor 9; treatment of the hydrogen sulphide output by step 2, at 11, by a Claus process to produce sulphur, and various heat exchangers 12 to 14.
In the prior art, the residual gas from the nitrogen-washing step 6 is burned.
A process for production of carbon monoxide, which makes it possible in the particular case above to utilize the residual gas rich in carbon monoxide is known from document EP-A-0,092,770. The residual gas is then used as feed mixture for the carbon monoxide production process. One of the processes described in that patent application includes a step of washing with methane in order to separate the hydrogen from a mixture also comprising methane, carbon monoxide and nitrogen. The condensate thus produced is distilled in a first distillation column in order to separate methane from the rest of the mixture. The gas from the head of this first column, mainly containing carbon monoxide, is distilled in a second distillation column in order to extract the nitrogen and the hydrogen remaining, the pure carbon monoxide being drawn off from the base of this column.
The carbon monoxide is then used as a coolant for the head condensers of the first and second columns.
A process for production of carbon monoxide from the residual gas, rich in carbon monoxide, output by a nitrogen-washing unit is also described in Patent Application EP-A-0 676 373.
To date, it has thus been possible to produce carbon monoxide by using the residual gas from a nitrogen-washing unit as the feed mixture for a separate carbon monoxide production unit, thus utilizing this residue which in the past was simply burned.